1. Field of the Invention
This invention relates to a magnetic recording medium on which information is recorded by the orientation states of magnetization in a magnetic material and information is reproduced using a light beam. The invention has particular, although not exclusive, relevance to a magneto-optical recording medium for use in domain wall displacement reproduction.
2. Description of the Related Art
Known magnetic recording media are being put to practical use as rewritable recording media. In particular, a magneto-optical recording medium, on which information is recorded by forming magnetic domains in a magnetic thin film utilizing the thermal energy of a semiconductor laser, the information being reproduced utilizing the magneto-optical effect, is expected to be used as a large capacity and rewritable medium on which it is possible to carry out recording of high density.
In recent years, due to the increasing digitalization of motion pictures, larger capacity recording media are required leading to a need to improve the recording density of these magnetic media.
In general, the line recording density of an optical recording medium strongly depends on the laser wavelength of the reproducing optical system and the aperture number NA of the objective lens used to focus the laser beam on the medium. In particular, the spatial frequency of the recording pit which is possible to reproduce is limited to about 2 NA since the beam waist is necessarily determined by the laser wavelength and the aperture number NA. Accordingly, it has been necessary to make the laser wavelength of the reproducing optical system shorter or to make the aperture number NA of the objective lens bigger in order to realize a high recording density in conventional optical recording media. However, making the laser wavelength shorter is not easy as this causes some problems in terms of generation of heat, efficiency of the laser device and the like. Furthermore, making the aperture number NA of the objective lens bigger causes the problem that the requirement of mechanical alignment become more strict because the depth of focus becomes shallow.
Therefore, various super resolution techniques, in which the recording density is improved by devising a structure of recording medium or a reproducing method without changing the laser wavelength and aperture number NA, are being developed. For example, JP-A-3-93058 describes a signal reproducing method in which the signal recorded on a multiple layer recording medium composed of a reproducing layer and a recording storage layer magnetically coupled with each other, is readout by transferring the recorded signal into the heated region of the reproducing layer by irradiating the reproducing layer with laser light after orienting the magnetization of the reproducing layer.
In this method, the region heated by the laser light to the transfer temperature is smaller than the diameter of the reproducing laser light spot. Accordingly, it is possible to reproduce a signal of spatial frequency greater than 2 NA because intersymbol interference is reduced when reproducing.
However, in this reproducing method, there is a disadvantage that the amplitude of the reproducing signal declines and a sufficient signal is difficult to obtain since the effective signal detecting area is smaller than the reproducing laser light spot diameter. As a result, it has been difficult to achieve a higher density than the recording density determined by the diffraction limitation of the optical system because it is difficult to make the effective signal detection area smaller.
To avoid the above problem, the present inventor has already proposed the magnetic recording medium and reproducing method described in JP-A-6-290496 in which it is possible to reproduce a signal of recording density greater than the resolution of the optical system without a decline in the amplitude of the reproducing signal. This is achieved by displacing the domain wall existing at the boundary portion between the recording marks towards the peak of the temperature gradient produced by the reproducing light spot and detecting the displacement of the domain wall.
However, in the method described in JP-A-6-290496, the peak of the temperature gradient is formed inside the reproducing light spot when forming the temperature gradient by heating the medium by the reproducing light beam itself. Therefore, both domain wall displacements from the front and from the rear of the relative direction of movement of the light spot to the medium are detected by the reproducing light beam. It has been difficult to obtain better signals because either domain wall displacement produces noise. Therefore, it is necessary to provide another light beam besides the reproducing light beam to form the predetermined temperature distribution, thus complicating the reproducing apparatus.
In JP-A-9-235885, it is proposed that the domain wall displacement from the rear of the light spot in the direction of relative movement of the light spot be restrained by applying a reproducing magnetic field. However, this complicates the reproducing apparatus because of the need to provide a magnetic field applying means.